What you’re seeing here are sand dunes on Mars. This region is in the center of a large crater at mid-north latitude on Mars, a couple of hours past local noon, and with a resolution of 50 cm (18 inches) per pixel. Sand dunes are common in crater beds, where the wind can blow steadily across the surface and sculpt the ever-present sand into those flowing sculptures.

But what this picture so spectacular are the graceful blue-gray swirls arcing across the dunes. These are caused by dust devils, which are a bit like mini-tornadoes. If the ground gets heated, rising air can punch through cooler air above it. This starts up a convection cell, with warm air rising and cool air sinking. If there is a horizontal wind the cell can start spinning, creating a vortex like a dust devil. I’ve seen hundreds of these on Earth, and they are wonderful and mesmerizing to watch.

The important thing to note here is that the sand in the craters of Mars is actually dark grey in color, since it’s made of basalt. The reason it looks red in pictures is because covering the sand is a thin layer of much finer dust, and the dust is what’s red. When a dust devil moves over the Martian surface, it can pick up the very light dust particles, but not the heavier sand grains. So those blue-grey swirls are tracks where the dust devil has vacuumed up the dust, revealing the darker sand underneath. If you look carefully in the tracks, you can see the sand dune ripples are undisturbed. Only the dust is gone.

HUGE IMAGE

(via Bad Astronomy – The artwork of Martian landscape)

The first attempts to reach Mars (1960) and Venus (1961) failed, yet triumph followed quickly. Of the nearly 200 solar, lunar, and interplanetary missions depicted on this map, most have been Earth’s closest neighbors. As rocketry, navigation, and imaging have become ever more capable and reliable, the planets and many of their moons have been examined in detail. The New Horizons mission to Pluto is under way, as is the MESSENGER mission to Mercury. Others not yet launched, perhaps not yet dreamed, await.

BIG BEAUTIFUL MAP

(via National Geographic – Fifty Years of Exploration)